In most industrialized nations of the world, cardiac disease is the leading cause of death. Of the many deaths associated with cardiac disease, ventricular arrhythmia is a significant contributor to sudden death in this patient population. Fortunately, studies and work have shown that an otherwise fatal ventricular arrhythmia may be reversed with a timely delivery of a high voltage electrical shock, referred to as a countershock, to the myocardium of the heart in an attempt to restore, at least temporarily, the natural electrical rhythm of the heart.
As a background for understanding the present invention, it is helpful to understand the medical basis for why the electrical and muscular activity of the ventricles of the heart muscle can lead to the death of a patient. Ventricular arrhythmia have been described in several ways, but there are generally considered to be three types of ventricular arrhythmias: ventricular fibrillation, high rate ventricular tachycardia, and low rate ventricular tachycardia.
Ventricular fibrillation is described as the chaotic discharge of electrical and myocardial muscular activity dispersed throughout the ventricles that results in a mechanically non-functioning heart. Visually, ventricular fibrillation appears as if the myocardium were a mass of quivering worms. Onset of ventricular fibrillation results in instantaneous loss of cardiac output, as both blood pressure and perfusion pressures drop to zero. A patient so inflicted will lose consciousness within seconds and, if a perfusing pulse pressure is not reestablished within approximately three minutes, significant brain injury begins to occur that rapidly deteriorates to brain death within approximately five minutes.
Ventricular tachycardia is distinguishable from ventricular fibrillation on the basis of what appears to be an organized electrical activity of the myocardium. However, this electrical activity is so rapid that it exceeds the heart's ability to adequately pump blood. Cardiac output is dependent upon the degree of preload and filling the heart can achieve prior to the contraction event that produces the cardiac output. If the heart rate is too fast and exceeds the preload and filling time, the heart will have nothing to pump out. The end result of a high rate ventricular tachycardia can range from significantly lowered blood pressure and perfusion pressures to the absence of any pressures. Once again, a patient so inflicted will lose consciousness and, if the loss of perfusion pressure is significant, irreversible changes within the brain will occur due to loss of adequate oxygenation just as in the ventricular fibrillation scenario with permanent brain injury and ultimately brain death ensuing within minutes after onset of the high rate ventricular tachycardia.
Low rate ventricular tachycardia electrically appears very similar to the high rate ventricular tachycardia except for the slower electrical response time of the ventricles. Due to this lower rate, filling times are lengthened and some amount of blood is able to reach the ventricles in order to be pumped out on the next contraction cycle. The end result of low rate ventricular tachycardia is that the patient is able to sustain some blood pressure and perfusion pressure, but less than normal. Unfortunately, if a low rate ventricular tachycardia is allowed to persist it can, and most likely will, deteriorate to high rate ventricular tachycardia and ultimately ventricular fibrillation. A patient suffering from a low rate ventricular tachycardia may not be in imminent danger of dying, as are patients with ventricular fibrillation or even high rate ventricular tachycardia, but if there is not adequate intervention within minutes, the natural course of this disease process is to deteriorate to the more highly fatal arrhythmias.
Most people are familiar with the use of external defibrillators in an emergency situation where a patient with a ventricular arrythmia is treated by delivering a powerful electrical shock or countershock to the heart via external electrodes placed on the patient's chest. It should be noted that, while countershock therapy may prove efficacious in reversing the electrical arrhythmia portion of a ventricular arrhythmia, countershock therapy does not treat the underlying cardiac disease process that triggered the ventricular arrhythmia. Instead, countershock therapy provides a needed stopgap measure that allows additional time for medical responders to support a patient until further medical measures can be instituted to treat the underlying cardiac disease process.
More recently, implantable cardioverter defibrillators or ICD systems have been developed that automatically detect the onset of a ventricular arrhythmia and, in response, deliver a countershock to the myocardium via two or more implanted electrodes. Presently, there are several different ICD systems which have received device approval from the Federal Drug Administration, the PCD.TM. device, available from Medtronic, Inc., of Minneapolis, Minn., the Cadence.RTM. device, available from Ventritex, Inc., of Mountain View, Calif., and the Ventak.RTM. PRx.TM. device, available from Cardiac Pacemakers, Inc., St. Paul, Minn. These ICD systems have the obvious advantage of not requiring an emergency medical team with an external defibrillator in order to treat a patient with a ventricular arrythmia. In addition, because the electrical shock is delivered internally, rather than through the epidermal layer of skin, the electrical shock can be much less powerful and still perform its intended function.
The primary components of existing ICD systems include an automatic monitoring and detection mechanism, a capacitor system, a battery system and control circuitry for detecting a ventricular arrhythmia and controlling delivery of a high voltage capacitive discharge electrical countershock in response by charging and then discharging the capacitor system. The existing ICD systems are capable of delivering a maximum countershock of up to 700 to 750 volts having a total energy of between 31 to 44 joules. At the time an ICD system is implanted in a patient, the attending physician will empirically determine a minimum defibrillation threshold for the patient, and will program the charging voltages for the countershocks to be delivered as part of a therapy regimen within the range of maximum voltages allowed by the device.
In order to meet these discharge requirements while packaging the ICD systems within as small a container as is physically possible, design considerations must keep in mind the specifications on upper limits of size when making component choices. One such component that contributes significantly to the minimum size attainable by an ICD system is the capacitor system. The aluminum oxide electrolytic capacitor to date has proven to be the best capacitor technology for use in ICD systems. Due to the nature of the aluminum oxide dielectric, however, electrolytic capacitors are limited to maximum rated charging voltages in the range of approximately 350-375 volts. Beyond 375 volts, electrolytic capacitors begin to suffer from significant leakage current across the dielectric. This leakage current increases rapidly as the voltage is increased and charging of the electrolytic capacitor will cease when the leakage current equals the charge current. The advantage of electrolytic capacitors is that catastrophic breakdown due to overcharging is a rare event, the trade off being that an electrolytic capacitor cannot be charged beyond what the leakage current will allow. To that end, to maintain safe reproducible performance, present ICD systems utilize two electrolytic capacitors discharged in series to deliver the high voltage shock to the myocardium having a maximum voltage of approximately 700-750 volts.
By utilizing two electrolytic capacitors charged and then discharged in series, existing ICD systems avoid the disadvantage of wasted energy from leakage currents while still achieving the high voltage necessary to achieve successful defibrillation. This configuration also provides energy storage in as small a volume as is possible. It should be mentioned that other capacitor technologies and substantially higher voltages have been proposed in the prior art. For example, the original ICD patent, U.S. Pat. No. 3,614,945 issued to Mirowski proposed to charge a capacitor to 2,500 volts. This energy level is achievable using polymer film dielectric capacitor technology. With polymer films, a capacitor's voltage rating is proportional to the polymer film thickness. Consequently, such a capacitor charged to 2,500 volts would necessarily have a considerable and unmanageable volume and would not be suitable for use in a practical ICD system. In fact, no such implantable defibrillator system product has ever been developed using polymer film capacitors.
Existing ICD systems in general operate in accordance with the following functional steps. After implantation, the ICD system is in a monitoring mode, vigilant for the onset of a ventricular arrhythmia. If a ventricular arrhythmia is detected, existing ICD systems will generally follow a confirmation paradigm which lasts approximately five seconds to ensure that the initial detection is accurate. With confirmation, the existing ICD systems will begin charging the high voltage capacitor system to a preprogrammed voltage level to deliver an electrical countershock therapy for the detected arrhythmia. This process takes between 5-20 seconds depending on the energy setting and the battery capacity. Typically, for maximum voltage charges, the charging time is approximately 12-15 seconds. With completion of the charging, the diagnosis of a ventricular arrhythmia is typically reconfirmed, a process which can takes an additional 1-5 seconds. Following reconfirmation of a ventricular arrhythmia, the countershock is delivered. After countershock delivery, it can take anywhere from two and a half to ten seconds for the monitoring circuitry of the ICD system to settle down and restart the monitoring process to check for success or failure of the countershock therapy. Thus, the amount of time needed to deliver a single electrical countershock by existing ICD systems can take upwards of thirty or more seconds.
In the event that the countershock is not successful in reversing the cardiac arrhythmia, existing ICD devices repeat this cycle up to a maximum of five times. Depending upon the charging voltages programmed at the time of implantation, it is common that the first countershock of a therapy regimen will be programmed to deliver a voltage of less than about 700 volts and that the four subsequent countershocks of the therapy regimen will be delivered at the maximum rated charging voltage for the device. The reason why the first countershock may be programmed to be delivered at a lower voltage is that it has been shown that a countershock is more effective and requires less energy the earlier it is instituted in response to the detection of a cardiac arrhythmia. It will be seen that the limit of five countershocks per therapy regimen for existing ICD systems is in recognition of the fact that permanent brain injury begins to occur at approximately three minutes after onset of a lethal ventricular arrythmia. Existing ICD systems cannot practically deliver more than five countershocks in less than about three minutes.
Although existing ICD systems provide a useful mechanism for temporarily treating cardiac arrhythmias and thereby increase the survival chances of the patient, the existing manner in which the ICD systems are operated provides for an effective limit of five electrical countershocks per therapy regimen. Each countershock is delivered approximately every 30 seconds at a charging voltage not greater than the maximum rated charging voltage of the device. As a result, it is very common for physicians implanting the device to program the charging voltages for the second through fifth countershocks at the maximum rated charging voltage of the device. Unfortunately, the countershocks after the second or third countershock in this type of programmed therapy regimen have significantly less chance of success. For example, the fourth and fifth countershocks are identical to the preceding countershock, but are delivered at least 30 seconds later during which time the chances of the patient responding to electrical countershock therapy have continued to deteriorate.
Accordingly, it would be advantageous to provide an ICD system that could deliver a more efficacious programmed therapy regimen than the therapy regimens available on existing ICD devices. In addition, it would be advantageous if an ICD system could overcome the existing limitations imposed by the maximum charging voltages of electrolytic capacitor systems to further decrease the overall size of the ICD system.